1. Field of the Invention
The present invention relates to an accumulation type fuel injection system, in particular to sealing structure of an accumulation device of the accumulation type fuel injection system.
2. Description of Related Art
As one of accumulation type fuel injection systems, a common rail type fuel injection system for a diesel engine having a high-pressure supply pump and a common rail is known, for instance. The high-pressure supply pump is rotated by a crankshaft of the diesel engine so that the supply pump pressurizes fuel drawn from a fuel tank and discharges the high-pressure fuel. The common rail functions as a kind of surge tank for accumulating the high-pressure fuel discharged from the high-pressure supply pump.
The fuel is accumulated in the common rail at a relatively high pressure (a pressure 100 to 1000 times as high as the atmospheric pressure, or more). The accumulated fuel is supplied to combustion chambers of cylinders by injection through injectors mounted on the cylinders. A certain common rail as the accumulation device of this kind has structure in which a flow limiter is attached to the common rail as shown in FIG. 4. The flow limiter stops the fuel supply to the injector if the high-pressure fuel is injected from the injector excessively. In the common rail structure, the common rail is formed in a deformed shape so that a thickened screw portion, which is screwed and connected with the flow limiter, and a common rail main body are formed in a single piece. Since the common rail is formed into the deformed shape in a single piece, processing accuracy in perpendicularity between a central axis of the screw portion and a flat surface of a flat sealing portion of the common rail main body can be achieved easily and high-pressure sealing structure can be maintained.
As shown in FIG. 4, the flow limiter has a body formed with a screw portion screwed with the common rail, a valve member capable of moving in an axial direction in the body, and a spring for biasing the valve member toward the flat sealing portion. The flat sealing portion contacts a lower end surface of the valve member to define an initial position of the valve member so that a moving distance of the valve member in the axial direction is set to a predetermined distance. The valve member moves in the axial direction in accordance with an upstream and downstream pressure difference.
However, the common rail having the structure of the conventional technology is the deformed product formed by forging and the like. Therefore, there is a problem that production cost is relatively high.
A joined common rail can be employed as a countermeasure to the above problem. The joined common rail is made by forming the common rail main body and the thickened screw portion as separate parts and by joining the thickened screw portion to the common rail main body into the single common rail by welding and the like. The thickened screw portion and the common rail main body formed as the separate parts are joined with each other with heat by welding, in which both the thickened screw portion and the common rail main body are melted, or by blazing with blazing filler. Therefore, under some welding conditions or blazing conditions, there is a possibility that a central axis of a threaded portion of the thickened screw portion may not become perpendicular to the flat surface of the flat sealing portion, and the thickened screw portion may be joined in an inclined state. In this case, there is a possibility that the flat sealing portion may contact a sealing portion of the flow limiter unevenly. As a result, high-pressure sealing performance may be degraded.